M4 Nozzle/hopup shimming
This is a simple little mod, it's been discussed allot over the years. However I'm still seeing guns commonly with this issue on the field. And I'm surprised how often it's an issue with m4 based guns. Some other guns solved this with various design fixes, but allot of m4 based guns have this issue. If your not familiar with it give it a quick read and you might find a way to make your gun a bit more consistent.
So first things first if this is an M4 or a variant of an M4 I’d check the way the hopup meets up against the gearbox. Typically there is a small spring there which is supposed to help hold the hopup against the front of the gearbox which keeps compression between the hopup and nozzle extending from the gearbox. What happens is certain mags which are out of spec, or mag wells that are out of spec, can push the hopup chamber forward. Which causes a massive compression leak between the hopup and nozzle when fired. I’ve seen this cause a drop of over 100fps on some guns. Most seem to be 10-50fps depending on the magazine and how bad it is.
Diagnose, easiest way to tell is to reach into the mag well without a mag installed in the gun. See if you can move the feed tube forward towards the muzzle easily. As well look in the dust cover at the point where the hopup touches the mech box. Put a mag in the gun and wiggle the mag, you normally will see the hopup unit move off the mechbox. Any time that hopup isn’t snug to the mechbox your loosing FPS. That causes inconsistent shooting, and accuracy. Also it may explain some mags liking certain guns and others causing massive issues.
Solution, or the way I prefer to fix this is to remove the upper receiver from the gun. Pull the hopup/inner barrel from the upper. Then the only part needed are some small orings, just larger than the inner barrel. Take an oring and stretch it over the muzzle end of the inner barrel and slide it all the way to the hopup chamber. Then put the barrel back in the upper and put it back together. See if you can move the feed tube again like mentioned in the paragraph above. If it’s pretty snug IE very little movement. Your probably right and done. Note, you can get this too tight and can cause gearbox issues. You want this snug not tight, there is a fine line here but you want it snug enough mags aren’t going to move the hopup chamber away from the mechbox.
Once you figure out how many orings you need to get the pressure you’re done. You can remove the little spring if you want from the top of the hopup chamber. But typically I leave it there despite it not really doing anything anymore.
Hope that all makes sense, if guys are confused feel free to post and I can include pictures, or find a video likely of someone doing this on Youtube. Mostly I just wanted to make people aware that this is a simple mod that most anyone can do, that often works very well to make your M4 based gun more consistent especially with various magazines.
Take care!
Luke
